On his site, blogZero, Douglas Clifton shares his method for authenticating a Googlebot in both Perl and PHP. Basicslly, making sure that the bot hitting your server with the user agent containing Google is, in actuality, really from the search engine.

Following a tip from How to verify Googlebot. In a nutshell, it explains how to use the Unix shell program host to authenticate that an IP address copied from your Web server log file really is a Googlebot and not some email harvester (or whatever).

I decided to take this a step further and demonstrate how you can automate this procedure using a scripting language.

He looks at methods using both programming languages and decides on showing off the PHP version first. It uses a simple method, made even easier by two of PHP's internal functions, gethostbyname and gethostbyaddr. The Perl version relies less of the "higher level" that PHP's functions use and opts for making an actual Socket object to handle the incoming request.